In an information recording and reproducing apparatus such as a hard disk drive (HDD), data can be recorded and reproduced by moving a recording/reproducing head to a target track on the surface of a rotating disk. Reducing the track pitch on a magnetic disk is one way to improve recording density. With the reduction in the track pitch, however, the distance between data written on adjacent tracks becomes smaller. As a result, errors are easily generated even when the write and read positions of the magnetic head are deviated only slightly.
Generally, during a servo track write (STW) process, the position of the magnetic head is measured and servo information is written on the magnetic disk using an external actuator for positioning the arm of an internal actuator of the magnetic disk apparatus. The servo information is typically written under conditions where a vibration is generated on a spindle motor of the hard disk drive. Consequently, the servo information is written at a position displaced from the ideal circle (locus of head which is intrinsically considered as desirable locus). As a result, the servo information includes an error element that may cause fluctuation of the magnetic head.
Moreover, there is also a problem of fluctuations in the transition of magnetization being generated in the written servo information. The surface of a magnetic disk is formed with aggregation of magnetic particles. These magnetic particles are not arranged orderly based on the size. Therefore, a boundary between signals is not straight and a large area is required for the signal for one bit.
However, in order to record data in narrower area, i.e., to record data in higher density, the boundary between signals must be linear. When high density recording is performed on the disk having these magnetic particles, the servo signal cannot be written smoothly. If the servo signal is not written smoothly, the SN-pole directions of the magnetic particles are not defined in a constant direction, and thereby, a weak area is generated in the magnetization of the servo signal. Therefore, the magnetic head cannot read the servo signal correctly and displacement and fluctuation are generated when the magnetic head follows the tracks.
A proposal has been made to write correcting data on a magnetic disk for correcting an error of a servo signal resulting from a disturbance of a position information and a magnetic fault of the magnetic disk when the servo signal is formed. The aim is to realize the positioning of the magnetic head with higher accuracy when data is read and written on the basis of the correcting data.
Japanese Patent Application Laid Open No. H03-263662 proposes a technology for writing an error signal indicating an error of the servo signal to one area or both areas before and after the serve frame of the magnetic disk in order to read such error signal in the on-track timing. Position or servo information may include an error element due to the influence of vibration resulting from the rotation of the magnetic disk at the time of forming the servo signal. The positioning error creates a large influence in high density tracks. Therefore, the error signal indicating an error of the servo signal is written to the area before the servo frame of the minimum unit in the circumferential direction of the servo signal, or to the area after the servo frame, or to both areas before and after the servo frame. Since the error information is read with the magnetic head in the on-track timing, accurate positioning of the magnetic head is said to be realized for the target track by correcting an error of the servo signal.
Japanese Patent Application Laid Open No. S60-117461 proposes a technology for improving a read margin. More specifically, an original servo signal read with a servo head is input to a servo signal reproducing circuit and is then converted to the servo signal. However, if a magnetic fault is found in the original servo signal, a positional deviation is generated when the position signal of head is generated only with the servo signal obtained from the original servo signal. Therefore, while the tracking is performed on the basis of the servo signal including a positional deviation, the correcting signal is written on the magnetic disk with a servo head. In this case, since it is sometimes assumed that a fault is found in the track on which the correcting signal is written, multiple correcting signal is written, or the correcting signal is given the parity. Accordingly, the correcting signal is written before the part which requires a correction.
It is also known to record the correcting data in each track, as illustrated in FIG. 15, to an area 70 provided immediately after each servo frame 50, and this correcting data is alternately allocated for each servo frame through a division into a postcode area 41 for recording and a postcode area 42 for reproducing. In this case, the postcodes 41, 42 allocated to the adjacent tracks are isolated through small gap. The gap between the adjacent codes becomes even smaller when the distance (track width) between the adjacent tracks is reduced through high density in recording capacity.
FIG. 16 illustrates a fluctuation in the tracking of the magnetic head 20. If there is no allowance for providing an interval between the postcodes 41 or postcodes 42, a fluctuation is generated in the magnetic head 20 because of the vibration generated by a motor of the apparatus. Such fluctuation results in data intended for a target track 61 being over-written on the adjacent track 62 during the write operation of the postcodes 41. Such over-writing generates noise during the read operation and such noise will be output as a read error.
In addition, for allocation of the postcodes 41, 42, information indicating the start of the postcode must be added for each postcode. Therefore, when a large number of postcodes are allocated, the recording capacity of the user data area is reduced.